The hard carbon film is a rigid film called diamond-like carbon (hereinafter referred to as DLC. A film/layer composed mainly of DLC is called DLC film/layer.). In addition the hard carbon film has various names such as a hard noncrystalline carbon, amorphous carbon, hard amorphous carbon, i-carbon, and diamond-like carbon, but these terms are not clearly distinguished from one another.
As the essential quality of the DLC for which the above-described terms are used, the DLC has a structure intermediate between diamond and graphite. The DLC has a high hardness comparable to diamond and is excellent in its wear resistance, lubricity, thermal conductivity, and chemical stability. Therefore the DLC is utilized as a protection film of dies, tools, wear-resistant machine parts, abrasive materials, sliding members, and magnetic optical component parts. As methods for forming the DLC film, a physical vapor deposition (hereinafter referred to as PVD) method such as a sputtering method and an ion-plating method and a chemical vapor deposition (hereinafter referred to as CVD) method are adopted. For example, a DLC film obtained by a filtered arc method used in an arc ion-plating is known (see patent document 1).
Normally the DLC film generates a very large internal stress when it is formed and has a high hardness and Young's modulus but a very small deformability. Thus the DLC film has defects that it has a low adhesion to a base material and is liable to peel therefrom. As means for improving the adhesion to the base material, two methods are listed: (1) a method of controlling the stress of the film and (2) a method of forming an intermediate layer between the base material and a carbon film. But these arts have problems shown below. The actual situation is that improvement for these arts is desired. In the above-described methods, from the standpoint of joining the base material and DLC film with each other with an intermediate layer, serving as a gluing layer, which has a property intermediate between the tissues and mechanical properties of the base material and DLC film. Thus the intermediate layer containing a hard and brittle material is adopted. But when a thick film having a thickness of several micrometers is formed and when a hard film, containing a large amount of a diamond component, whose hardness exceeds 40 GPa is formed, there arises a problem of defective adhesion owing to a very large stress in the DLC film formed by the PVD method or the CVD method.
To solve this problem, a DLC hard multilayer film formed body in which the uppermost surface layer consists of the DLC film showing adhesion not less than 50N in a scratch test is known (see patent document 2). This art relates to the DLC hard multilayer film formed body having a two-layer structure composed of the uppermost surface layer consisting of the DLC film and the intermediate layer, disposed between the base material and the uppermost surface layer, which consists of the amorphous layer containing carbon and not less than one kind of metallic element selected from among W, Ta, Mo, and Nb. Description is made in the specification of the patent document 2 that in the DLC hard multilayer film formed body having the above-described film structure, a preferable adhesion of the DLC film to the base material consisting of cemented carbide such as WC—Co. But this art also has a problem to be solved, as described below.
In the above-described art, the cemented carbide such as the WC—Co is fundamentally used as the base material. Therefore in the case where WC—Co-based cemented carbide and an insulation material such as Si, Al2O3 are used as the base material, the intermediate layer is capable of securing a preferable adhesion to the base material. But when a ferrous material such as high-speed tool steel is used as the base material, the intermediate layer and the base material are not always compatible with each other. Thus this art has a problem that the adhesion between the intermediate layer and the base material is poor and that the DLC film is liable to peel from the base material. In addition the condition for forming the DLC film excellent it its wear resistance as the uppermost surface layer of the hard multilayer film formed body is not optimized and thus there is room for improvement.
As an art for improving the adhesion, the art of coating the base material consisting of a ferrous material having a low hardness with the DLC film with a high adhesion is known. According to the description, even though the DLC film is comparatively thickly formed, this art allows the DLC film to display excellent adhesion to the base material (see patent document 3). This art relates to the hard multilayer film formed body including the uppermost surface layer composed mainly of the DLC, the intermediate layer, and the base material consisting of the ferrous material. The intermediate layer has a four-layer structure.
(1) The first layer consisting of the metallic layer of Cr and/or Al
(2) The second layer consisting of the mixed layer of the metal of Cr and/or Al and not less than one kind of the metal selected from among the group of W, Ta, Mo, and Nb
(3) The third layer consisting of not less than one kind of the metal selected from among the group of W, Ta, Mo, and Nb
(4) The fourth layer consisting of the amorphous layer containing carbon and not less than one kind of the metal selected from among the group of W, Ta, Mo, and Nb
In the specification of the patent document 3, description is further made as follows: It is preferable that the second layer has the graded layer in which the content of the Cr and/or the Al stepwise or continuously decreases toward the uppermost surface layer. It is preferable that the fourth layer has the gradient composition in which the content of not less than one kind of the metal selected from among the group of W, Ta, Mo, and Nb stepwise or continuously decreases toward the uppermost surface layer. Description is further made that it is possible to use a compound containing WC as its main component instead of not less than one kind of the metal selected from among the group of W, Ta, Mo, and Nb which are the components of the second, third, and fourth layers.
To form the DLC film excellent in its adhesion, a method for forming the hard multilayer film having the two-layer structure by utilizing unbalanced magnetron sputtering (hereinafter referred to as UBMS) has been proposed (see patent document 4).